Efficiency In D.C Machines - MCQs with Answers

Efficiency In D.C Machines - MCQs with answers

1. If % η _m is the mechanical efficiency of a d.c. machine and % η _e is the electrical efficiency. Then the overall efficiency of d.c. machine is given by

A. % η _m + % η _e
B. % η _m X % η _e
C. (% η _m + % η _e) / % η _m
D. (% η _m + % η _e) / % η _e

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Q2. The current flowing through the armature of a d.c. shunt machine at maximum efficiency is given by

A. √(P_i / R_a)
B. √(R_a / P_i)
C. √(P_i / R²_a)
D. √(R_a / P²_i)

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Q3. A 120 V shunt generator running at 850 rpm has its armature and shunt field resistance of 0.15 ohm and 50 ohm respectively. It supplies 200 lamps each rated at 60 W, 100 V. The friction and windage and core loss of the machine is 400 W. its armature copper loss on full load and shunt field loss is

A. 2156.7 W, 200 W
B. 2232.6 W, 200 W
C. 2156.7 W, 240 W
D. 2232.6 W, 240 W

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Q4. A d.c shunt generator delivers 190 A at a terminal voltage of 220 V. The copper losses and stray losses are 2000 W and 1000 W respectively. The efficiency of the generator is

A. 91.35 %
B. 92.60 %
C. 93.30 %
D. 94.23 %

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Q5. A 4 pole d.c. shunt generator having a wave winding supplies 45 lamps, each of 50 W at 100 V. The armature and field resistance are 0.15 ohm and 50 ohm respectively. The current in armature conductor is

A. 11.25 A
B. 11.50 A
C. 12.25 A
D. 13.50 A

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Q6. A 120 V d.c. shunt motor runs at speed of 1200 rpm. When the motor is operated unloaded but an additional resistance of 4 ohm is connected in series with the shunt field, then the speed increases and reaches to 1370 rpm with the same terminal voltage. The value of series resistance is

A. 28.23 ohm
B. 32.48 ohm
C. 35.72 ohm
D. 36.82 ohm

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Q7. A 240 V, 17 kW d.c. shunt motor draws an armature current of 80 A at full load. The armature and shunt field resistances are 0.2 ohm and 195 ohm respectively. The rotational losses and efficiency of motor at full load is

A. 87.20 %
B. 89.32 %
C. 89.67 %
D. 90.03 %

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